Definition/Description

Injuries to the ACL are relatively common knee injuries among athletes.[1] They occur most frequently in those who play sports involving pivoting (e.g. football, basketball, netball, soccer, European team handball, gymnastics, downhill skiing). They can range from mild (such as small tears/sprain) to severe (when the ligament is completely torn). Both contact and non-contact injuries can occur, although non-contact tears and ruptures are most common. It appears that females tend to have a higher incidence rate of ACL injury than males, that being between 2.4 and 9.7 times higher in female athletes competing in similar activities.[2][3][4][5][6]

Most common are the non-contact injuries, caused by forces generated within the athlete’s body while most other sport injuries involve a transfer of energy from an external source.[10] Approximately 75% of ruptures are sustained with minimal or no contact at the time of injury.[11] A cut-and-plant movement is the typical mechanism that causes the ACL to tear, that being a sudden change in direction or speed with the foot firmly planted. Rapid deceleration moments, including those that also involve planting the affected leg to cut and change direction, have also been linked to ACL injuries, as well as landing from a jump, pivoting, twisting, and direct impact to the front of the tibia.[11]

Women are three times more prone to have the ACL injured then men, and is thought to be due to the following reasons:[12]

Smaller size and different shape of the intercondylar notchA narrow intercondylar notch and a plateau environment are risk factors of predisposing female nonathletes with knee OA to ACL injury aged 41-65 years. [13] (Level 3b)

Wider pelvis and greater Q angleA wider pelvis requires the femur to have a greater angle towards the knee, lesser muscle strength provides less knee support, and hormonal variations may alter the laxity of ligaments.[14][15]

Shoe surface interfaceThe pooled data from the three studies suggest that the odds of injury are approximately 2.5 times higher when higher levels of rotational traction are present at the shoe-surfaceinterface. [17] (Level 1a)

Neuromuscular factors

Risk factors for ACL injuries include environmental factors (e.g. high level of friction between shoes and the playing surface) and anatomical factors (e.g. narrow femoral intercondylar notch). The injury is characterized by joint instability, which is associated with both acute dysfunction and long-term degenerative changes, such as osteoarthritis and meniscal damage. Knee instability leads to decreased activity, which can lead to poor knee-related quality of life. [18][9] (Level 5) [19] (Level 2b). The risk factors for ACL injury have been considered as either internal or external to an individual. External risk factors include type of competition, footwear and surface, and environmental conditions. Internal risk factors include anatomical, hormonal and neuromuscular risk factors. [8][20][21] (Level 5)

External risk factors [8][20] (Level 5)Competition in games versus practice Very little is known about the effect of type of competition on the risk of an athlete suffering ACL injury. Myklebust et al reported that athletes are at a higher risk of suffering an ACL injury during a game than during practice. This finding introduces the hypothesis that the level of competition, the way in which an athlete competes, or some combination of the two increases an athlete’s risk of suffering an ACL injury.

Footwear and playing surface Although increasing the coefficient of friction between the sports shoe and playing surface may improve traction and sports performance, it also has the potential to increase the risk of injury to the ACL. Lambson et al found that the risk of suffering an ACL injury is greater in football athletes who have boots with a higher number of cleats and an associated higher torsional resistance at the foot-turf interface. Olsen et al reported that the risk of suffering an ACL injury is greater in female team handball athletes who compete on artificial floors that have a higher torsional resistance at the foot-floor interface than in those who compete on wood floors. This relationship did not exist for male athletes.

Protective equipment There is controversial about the use of functional bracing to protect the ACL-deficient knee. Kocher et al studied professional skiers with ACL-deficient knees and found a greater risk of knee injury in those who did not wear a functional brace than in those who did use a brace. McDevitt et al performed a randomized controlled study of the use of functional braces in cadets attending the US military academies who underwent ACL reconstruction. At the 1-year follow-up the use of functional bracing did not affect the rate of ACL graft re-injury. It is important to point out that there were only three injuries among those in the unbraced group and two injuries in the braced group.

Meteorological conditions For sports that are played on natural or artificial turf, the mechanical interface between the foot and playing surface is highly dependent on the meteorological conditions. However, very little is known about the effect of these variables on an athlete’s risk of suffering an ACL injury. Orchard et al reported that non-contact ACL injuries sustained during Australian football were more common during periods of low rainfall and high evaporation. This work introduces the hypothesis that meteorological conditions have a direct effect on the mechanical interface (or traction) between the shoe and playing surface, and this in turn has a direct effect on the likelihood of an athlete suffering an ACL injury.

Internal risk factors [8][20] (Level 5)Anatomical risk factors Abnormal posture and lower extremity alignment (eg, hip, knee and ankle) may predispose an individual to ACL injury by contributing to increased ACL strain values; alignment of the entire lower extremity should therefore be considered when assessing risk factors for ACL injury. Unfortunately, very few studies have studied alignment of the entire lower extremity and determined how it is related to the risk of ACL injury. Most of what is known has come from investigations of specific anatomical measures.

Biomechanics of Injury

As 60-80% of ACL injuries occur in non-contact situations, it seems likely that appropriate prevention efforts are warranted. Cutting or sidestep maneuvers are associated with dramatic increases in the varus-valgus and internal rotation moments. The ACL is placed at greater risk with both varus and internal rotation moments. The typical ACL injury occurs with the knee externally rotated and in 10-30° of flexion when the knee is placed in a valgus position as the athlete takes off from the planted foot and internally rotates with the aim of suddenly changing direction (as shown by the figure below).[22][23] The ground reaction force falls medial to the knee joint during a cutting maneuver and this added force may tax an already tensioned ACL and lead to failure. Similarly, in landing injuries, the knee is close to full extension. High-speed activities such as cutting or landing maneuvers require eccentric muscle action of the quadriceps to resist further flexion. It may be hypothesized that vigorous eccentric quadriceps muscle action may play a role in disruption of the ACL. Although this normally would be insufficient to tear the ACL, it may be that the addition of valgus knee position and/or rotation could trigger an ACL rupture.[24]

The athlete could be off balance, held by an opponent, avoiding collision with an opponent, or have adopted an unusually wide foot position. These perturbations contribute to this injury by causing the athlete to plant the foot so as to promote unfavorable lower extremity alignment; this may be compounded by inadequate muscle protection and poor neuromuscular control.[26] Fatigue and loss of concentration may also be a factor. What has become recognized is that unfavourable body movements in landing and pivoting can occur, leading to what has become known as the 'Functional Valgus' or 'dynamic valgus' knee, a pattern of knee collapse where the knee falls medial to the hip and foot. This has been called by Ireland (1996) as the 'Position of No Return', or perhaps it should be termed the 'injury prone position' since there is no proof that one cannot recover from this position.[27] Intervention programs aimed to reduce the risk of ACL injury are based on training safer neuromuscular patterns in simple maneuvers such as cutting and jump landing activities.[28]

Recently, a new hypothesis of how non-contact ACL injuries occur was presented. When valgus loading is applied, the medial collateral ligament becomes taut and lateral compression occurs. This compressive load, as well as the anterior force vector caused by quadriceps contraction, causes a displacement of the femur relative to the tibia where the lateral femoral condyle shifts posteriorly and the tibia translates anteriorly and rotates internally, resulting in ACL rupture. After the ACL is torn, the primary restraint to anterior translation of the tibia is gone. This causes the medial femoral condyle to also be displaced posteriorly, resulting in external rotation of the tibia. We can conclude that a valgus loading is a key factor in the ACL injury mechanism. At the same time, the knee rotated internally. A quadriceps drawer mechanism may also contribute to ACL injury as well as external rotation. [29] (Level 4)

Potential neuromuscular imbalances may be related to components of the injury mechanism: women have more ‘‘quadriceps dominant’’ neuromuscular patterns than men. Hamstring recruitment has been shown to be significantly higher in men than in women. The hamstring to quadriceps peak torque ratio tends to be greater in men than in women. Because of the likely injury mechanism, it is recommended that athletes avoid knee valgus and land with more knee flexion. [20] (Level 5)

Lower extremity valgus (knee abduction) loading and anterior tibial translation are likely to be involved in the mechanism. Future research should combine several research approaches to validate the findings such as video analysis, clinical studies, laboratory motion analysis, cadaver simulation and mathematical simulation. [30] (Level 4) [21] (Level 5)

The fibres of the ligament are completely torn (ruptured); the ligament itself has torn completely into two parts.

There is tenderness but not a lot of pain, especially when compared to the seriousness of the injury.

There may be a little swelling or a lot of swelling.

The ligament cannot control knee movements. The knee feels unstable or gives out at certain times.

There is also rotational instability as indicated by a positive pivot shift test.

No end point is evident.

Haemarthrosis occurs within 1-2 hours.

An ACL Avulsion occurs when the ACL is torn away from either the femur or the tibia. This type of injury is more common in children than adults. The term anterior cruciate deficient knee refers to a grade 3 sprain in which there is a complete tear of the ACL. It is generally accepted that a torn ACL will not heal.[32]

Medial Collateral Ligament Injuries

Associated injury to the MCL (Grade I-III) poses a particular problem due to tendency to develop stiffness after this injury. Most orthopaedic surgeons will first treat MCL injury in a limited-motion knee brace for a period of six weeks, during which time the athlete would undertake a comprehensive rehabilitation program. Only then would ACL reconstruction be performed or be treated.[34]

Bone Contusions and Microfractures

Subcortical trabecular bone injury (bone bruise) may occur due to the pressures exerted on the knee in traumatic injury and are especially associated with ACL rupture. Associated injuries of the menisci and the MCL tend to increase the progression of bone contusion.[35] The focal signal abnormalities in subchondral bone marrow seen on MRI (undetectable on rdiographs) are thought to represent microtrabecular fractures, haemorrhage and edema without disruption of adjacent cortices or articular cartilage.[36] Bone contusions may occur in isolation to ligamentous or meniscal injury.[37]

Occult bony lesions have been reported in 84-98% of the patients with ACL rupture.[35][38][39] The majority of these have lesions of the lateral compartment,[40] involving either the lateral femoral condyle, the lateral tibial plateau, or both. The boney bruising itself is unlikely to cause pain or reduced function.[41] Although the majority of bony lesions resolve, permanent alterations may remain. There is confusion in the literature as to how long these bony lesions remain, but it has been reported that they can persist on MRI for years.[42] Rehabilitation and the long-term prognosis may be affected in those patients with extensive bony and associated articular cartilage injuries. In the case of severe bone bruising it has been recommended to delay return to full weight-bearing status to prevent further collapse of subchondral bone and further aggravation of articular cartilage injury.[42]

Chondral Injury

Hollis et al suggested that all patients following traumatic ACL disruption sustained a chondral injury at the time of initial impact with subsequent longitudinal chondral degradation in compartments unaffected by the initial bone contusion, a process that is accelerated at 5 to 7 years’ follow-up.[43]

Tibial Plateau Fractures

A Tibial Plateau Fracture is a bone fracture or break in the continuity of the bone occurring in the proximal tibia affecting the knee joint, stability, and motion. The tibial plateau is a critical weight-bearing area located on the upper tibia and is composed of two slightly concave condyles (medial and lateral condyles) separated by an intercondylar eminence and the sloping areas in front and behind it. It can be divided into three regions: the medial tibial plateau (the part of the tibial plateau nearest the center of the body and contains the medial condyle), the lateral plateau (the part of the tibial plateau that is farthest away from the center of the body and contains the lateral condyle) and the central tibial plateau (located between the medial and lateral pleateaus and contains intercondylar eminence).

These fractures are also caused by varus or valgus forces combined with axial loading on knee and mostly occur with ACL injuries, rarely alone. The fracture of lateral tibial plateau is also called a Segond fracturewhich most commonly occurs with an ACL injury.

Posterolateral Corner Injury

The stability of the posterolateral corner of the knee is provided by capsular and noncapsular structures that function as static and dynamic stabilizers[44] including the lateral collateral ligament (LCL), the popliteus muscle and tendon including its fibular insertion (popliteofibular ligament), and the lateral and posterolateral capsule. Injuries to this region that result in posterolateral rotatory instability are usually associated with concurrent ligamentous injuries elsewhere in the knee.[45][46][47][48] High-grade posterolateral corner injuries are usually associated with rupture of one or both cruciate ligaments. Importantly, failure to address instability of the posterolateral corner structures increases the forces at the ACL and PCL graft sites, and may ultimately predispose to failure of the cruciate reconstruction.[49][50][51] (See also: Knee Rotary Instability)

Popliteal Cyst

Popliteal cysts, originally called Baker’s cyst, form when a bursa swells with synovial fluid, with or without a clear inciting etiology. Presentation ranges from asymptomatic to painful, limited knee motion. Management varies based on symptomatology and etiology.Popliteal cysts have been described as an interconnection between the knee joint and the bursa resulting from local fluid mechanics. Wolfe and Colloff stated that ‘there are two requirements for a cyst to form: the anatomical communication and a chronic effusion which opens this potential communication’. The pathophysiology of cyst formation has been attributed to trauma, arthritis and infection. Sansone et al. found that 44 of 47 popliteal cysts studied were associated with intra-articular lesions. The lesions include medial meniscal and anterior cruciate ligament tears, synovitis, chondral lesions, and total knee replacement. Intra-articular trauma, arthritis and infection result in knee effusions that lead to popliteal cyst formation. [52] (Level 5)

Popliteal cysts have been found in the posterolateral and posteromedial, thigh, between the gastrocnemius muscle and the deep fascia, and between the soleus and gastrocnemius muscles. Most occur within the posteromedial popliteal fossa between the gastrocnemius and deep fascia, as in the present study. Synovial fluid is produced by the synovial capsule through a rich meshwork of fenestrated micro vessels. The driving force for the continuous production of synovial fluid is the physiological osmotic gradient between the microvasculature of the synovium and the intra-articular space. The osmotic pressure of the intra-articular space draws fluid from the microvasculature according to the Starling forces. In the normal knee, intra-articular volume and pressure are minimized by the osmotic suction exerted by the synovial matrix. The synovial fluid is then drawn back into the veins and lymphatics of the synovium, from where it is pumped out by the articular motion of the knee. The pathological knee, associated with trauma, arthritis or infection, involves an increase in synovial fluid volume and pressure. An effusion occurs when the clearance of synovial fluid lags behind microvascular leakage. [53] (Level 3b)

Usually, in an adult patient, an underlying intra-articular disorder is present. In children, the cyst can be isolated and the knee joint normal.[54] (Level 5). Baker's cyst is less prevalent in a paediatric orthopaedic population than in an adult population. In children, it seems that Baker's cyst is seldom associated with joint fluid, meniscal tear, or anterior cruciate ligament tear. [55] (Level 3a)The study of Sansone et al. affirm that the popliteal cysts are associated with one, or more, disorders detected by MRI. The commonest lesions were meniscal (83%), frequently involving the posterior horn of the medial meniscus, chondral (43%), and anterior cruciate ligament tears (32%). [56] (Level 4)

Diagnostic Procedures

Radiographs of the knee should be performed when an ACL tear is suspected, including AP (anterior to posterior) view, lateral view, and patellofemoral projection. The standing AP weight-bearing view provides a way of evaluating the joint space between the femur and tibia.[57] It also allows for measurement of the notch width index which provides important predictive values for ACL tears.[58] The patellar tendon and height are measured on lateral radiograph. A tunnel view may also be helpful. The Merchant's radiograph view not only shows the joint space between the femur and patella but also helps to determine whether the patient has patellofemoral malalignment.[59]The presence of the following factors should be noted clearly during review of an x-ray:

Notch width index

Osteochondral fracture

Segond fracture

Bone bruise

The Notch width index is the ratio of the width of the intercondylar notch to the width of the distal femur at the level of the popliteal groove measured on a tunnel view roentgenogram of the knee. The normal intercondylar notch ratio is 0.231 ± 0.044. The intercondylar notch width index for men is larger than that for women. It was found that athletes with non-contact ACL injuries had a notch width index that was at least 1 standard deviation below the average, meaning that a person with an ACL injury is more likely to have a small notch width index compared to normal. It is measured with the help of a ruler placed parallel to joint line. The narrowest portion of the notch at the level of ruler is measured.[60] In more chronic ACL injuries, there may be intercondylar eminence spurring or hypertrophy, or patellar facet osteophyte formation.

Notch Width Index

Notch Width Index Measurement

This is also one of the reasons why women are more prone to ACL injuries compared to men. It has also been seen that the value of inner angle of the lateral condyle of femur was significantly higher in women athletes with ACL tear compared to those without. Value of width of intercondylar notch was statistically smaller in athletes with ACL tear, compared to those without. Also it was seen that the inner angle of lateral femoral condyle is a better predictive factor for ACL tears in young female handball players compared to intercondylar notch width.[61]

In more chronic ACL injuries, there may be interchondral eminence spurring or hypertrophy, patellar facet osteophyte formation, or joint space narrowing with marginal osteophytes. It is particularly important in skeletally immature patients to have plain radiographic assessment. This is because there is frequently a ligamentous avulsion in this age group.

A Bone bruise is usually present in conjunction with an ACL injury in more than in 80% of cases.[62] The most common site is over the lateral femoral condyle. The bone bruise is most likely caused by impaction between the posterior aspect of the lateral tibial plateau and the lateral femoral condyle during displacement of the joint at the time of the injury. The presence of bone bruise indicates impaction trauma to the articular cartilage.[63] Patients with bone bruises are more prone to develop osteoarthritis later.[64] Bone bruise can be seen most prominently in MRIs.

3. MRI

MRI has the advantage of providing a clearly defined image of all the anatomic structures of the knee. A normal ACL is seen as a well-defined band of low signal intensity on sagittal image through the intercondylar notch. With an acute injury to the ACL, the continuity of the ligament fibers appears disrupted and the ligament substance is ill defined, with a mixed signal intensity representing local edema and haemorrhage.[65]

MRI can diagnose ACL injuries with an accuracy of 95% or better.[66] MRI will also reveal any associated meniscal tears, chondral injuries, or bone bruises.

4. INSTRUMENTED LAXITY TESTING/ARTHROMETRIC EVALUATION OF KNEE

An adjunct to the clinical special tests in assessing anterior translation is the use of instrumented laxity testing. The most commonly cited arthrometer is the KT1000 (Medmetric, San Diego, California). The arthrometer provides an objective measurement of the anterior translation of the tibia that supplements the Lachman test in ACL injury. It can be particularly useful in the examination of acutely injured patients in whom pain and guarding may preclude evaluation. In such patients the Lachman and other tests can be difficult to perform accurately. The arthrometeric results can be used as a diagnostic tool to assess ACL integrity or as part of the follow up examination after ACL reconstruction.[67] The results of the KT1000 and its sibling. the KT2000 have been noted to be both reliable and accurate.[68]

Differential Diagnosis

The same characteristics for an ACL injury can be found at knee dislocations and meniscal injuries and collateral ligaments injury or posterolateral corner of the knee. Other problems that have to be considered are patellar dislocation or fracture, and a femoral, tibial or fibular fracture.

The differential diagnosis of an acute hemarthrosis of the knee due to ACL in addition to a major ligamentous tear would include meniscal tear or patellar dislocation or osteochondral fracture.

Differentiation can mostly be made based on a thorough examination with particular attention for the mechanism at the time of injury. An additional MRI scan can visualize the injury.

Can you spot the ACL and associated injuries in the MRI below?

Examination

The examination of ACL injury can be done in two ways:

Physical/Clinical examination

Examination under anesthesia and arthroscopy

Physical/Clinical Examination:

An organized, systematic physical examination is imperative when examining any joint. Immediately after the acute injury, the physical examination may be very limited due to apprehension and guarding by the patient. While inspecting, the examiner should look for the following:[70]

Overall alignment of the knee.

Severe distortion of the normal alignment may represent a fracture of the distal femur or proximal tibia or indicate knee dislocation.

Any gross effusion, which most commonly be present within a few hours after an ACL injury. Absence of an effusion does not mean that an ACL injury has not occurred; in fact, with more severe injuries that include the surrounding capsule and soft tissues, the hemarthrosis may be able to escape from the knee, and the degree of swelling may paradoxically be diminished. In addition, the presence of swelling and effusion does not guarantee that an ACL injury has occurred. According to Noyes et al, in the absence of bony trauma, an immediate effusion is believed to have a 72% correlation with an ACL injury of some degree.

Bony abnormality may suggest an associated fracture of the tibial plateau.

Palpation follows inspection and should begin with the uninvolved extremity. Palpation confirms the presence and degree of effusion and bony injury. Subtle effusions missed during inspection should be picked up by the careful manual examination. Palpation of joint lines and collateral ligaments can rule out a possible associated meniscus tear or sprained ligaments.

Periarticular tenderness should also be examined.

Assessing the patient’s range of motion (ROM) should be carried out to look for lack of complete extension, secondary to a possible bucket-handle meniscus tear or associated loose fragment.

Laxity testing should be done either with the special test or with the help of arthrometer.

Examination under anaesthesia and arthroscopy:

Arthroscopy combined with examination under anesthesia is an accurate way to diagnose a torn ACL. It may be indicated in the case whereby the diagnosis is suspected from the patient's history but is not evident on clinical examination. The main value of using arthroscopy on the basis of examination is to diagnose associate joint pathologic conditions such as meniscal tears or chondral fractures.[71][72]

Please see this page for additional information on assessment of the knee: Knee Examination

Physiotherapy Management

Prevention

Rates of noncontact ACL injury are higher among females than males. Several factors have been identified to explain this sex disparity. Gender differences have been found in motion patterns, positions, and muscular forces generated with various lower extremity coordinated activities. Anatomic and hormonal factors, such as a decrease ACL circumference, a small and narrow intercondylar notch width, a decrease joint laxity and a pre-ovulatory phase of menstrual cycle in females, have been discussed as increased risk factors for noncontact ACL injuries. [74] (Level 3b) [21] (Level 5)

However, modifying these particular risk factors is difficult if not impossible. In contrast, evidence indicates that neuromuscular risk factors are modifiable. Neuromuscular risk factors such as knee valgus position, muscular control (quadriceps and hamstrings muscular activation) and hip and trunk controls have been increasingly implicated in this injury etiology. [74] (Level 3b)[75] (Level 1a)

Given the importance of neuromuscular factors and the etiology of ACL injuries, numerous programs have aimed to improve neuromuscular control during standing, cutting, jumping, and landing.[76] The components of neuromuscular training are:

Balance training: balance exercises

Jump training – plyometrics: landing with increased flexion at the knee and hip

Strengthening that emphasizes proximal hip control mediated through gluteus and proximal hamstring activation in a close kinetic chain

Some studies suggest that plyometric and strengthening components were more important than balance training. But others studies suggest that both plyometric and balance training are effective at increasing measures of neuromuscular power and control. A combination of plyometric and balance training may further maximize the effectiveness. [77][80]

It is not only about WHAT we do, but also about HOW we do the exercises. The mode, length, frequency, and duration of neuromuscular interventions may vary. It can be very dependent of the individual. [74] But they are some clear dosages effects of neuro muscular training interventions. The higher the training volume (at least two times in a week for minimum 30 min), the better the effect. Also the higher the compliance, the better the effect.

Some studies revealed an age-related association between neuro muscular training implementation and reduction of ACL incidence. To optimized the reduction of ACL injury risk, we must initiate neuro muscular training before the onset of neuromuscular deficits and peak knee injury incidence in. Neuro muscular training may be most beneficial if initiate during early adolescence, before the period of altered mechanics that increase injury risk. [75][79][81]

Clinical Bottom Line

In order to provide the injured athlete with the best care, physiotherapists should have elaborate knowledge of anatomy and functioning of the ACL. The keystone to proper care of an ACL injury is to start from the correct diagnosis within the first hour of injury before the development of significant hemarthrosis. This should also include the detection and diagnosis of associated injuries.[89] Treatment for the injury and the return to play for an athlete depends completely upon the grade and associated injuries.

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